The hormonal control of cell death is currently the best established mechanism for creating sex differences in the nervous system. Nonetheless, very little is known about how hormones such as testosterone regulate neuronal cell death. This project will examine the cellular and molecular mechanisms underlying hormonally controlled cell death in three well-studied model systems: the spinal nucleus of the bulbocavernosus (SNB), the anteroventral periventricular nucleus (AVPv) of the hypothalamus, and the bed nucleus of the stria terminalis (BNST). Mice will be used throughout, to take advantage of the power of genetically manipulated strains. Members of the Bcl-2 family of proteins are crucial regulators of cell death in many neural areas. In Aim I we will ask whether Bcl-2 family members also regulate sexually dimorphic cell death by examining the expression and hormone regulation of pro-life (Bcl-2, Bcl-xL) and pro-death (Bax) molecules in the brain and spinal cord of neonatal mice. Testosterone decreases developmental cell death of the SNB and BNST, while increasing cell death in AVPv. We therefore predict that hormone treatments will differentially affect death-regulatory proteins in these neural areas. Bax knockout mice will be used in Aims 2 and 3 to test the hypotheses that Bax is required for the death of neurons in SNB, AVPv, and BNST; if so, then sex differences in neuron number in these regions will be eliminated in Bax knockouts. In addition to sex differences in overall cell number, AVPv and BNST exhibit large sex differences in the expression of neurotransmitters or neuropeptides. It is not known whether these differences are due to cell death, or the specification of neuronal phenotype. Cell death mutant mice will be used to discriminate between these hypotheses. Finally, in the SNB, testosterone likely regulates neuronal death indirectly, by controlling the availability of neurotrophic factors from target cells. Aim 4 will test whether a recently identified trophic factor rescues SNB cells of mice and, in so doing, alters the expression of Bcl-2 family members. Together, these studies will allow us to specify at a mechanistic level how hormones control neuronal cell death in neural systems. This work is relevant to understanding sex differences in susceptibility to human neurodevelopmental disorders and neurodegenerative diseases in adulthood.